We produce a part for an aerospace customer.  The part is titanium and requires machining, alkaline cleaning, press fit (titanium) plugs installed, electron beam welding and then subsequent stress relieve.

The material is AMS 4928 (6AL-4V).

The welding is performed in a vacuum.  There is discoloration (straw, light blue, deep blue) which appears to be vapor deposition.  There are over twenty welds to be performed and the chamber has to be vented and opened for each repositioning of the part.

The part is stress relieved at 990°F in a vacuum.  The color change is minor from the stress relieve.

We do perform an alpha case test on a sampling plan and have not found any occurances.

The parts are handled with clean white gloves from the alkaline clean stage until after heat treat.

The question is really about the discoloration from the welding.  What is the expectation of color from vapor deposition?

Is there a way to minimize the vapor deposition?

Thank you,

kidvb

Check your shielding gas purity and check gas dew point.

Oops, my apologies, I missed that welding is performed in a vacuum chamber.

What vacuum level are you using prior to welding?   

kidvb;
Here is an article you might find interesting regarding discoloration of TI form welding...


http://www.thefabricator.com/article/arcwelding/the-facts-on-welding-titanium

Your vacuum may be insufficient. You may also wish to vapor degrease as well as caustic clean.  

I agree with stanweld on both points- potential vacuum issue, but contamination may still be present after alkaline clean, especially if it not rinsed well.

Per AMS2680 3.7.1 for electron beam (EB) welding of titanium it is generally acceptable to have this bluish vapor deposition, as long as it is not in the weld joint itself.
Per the spec, the weld bead and the adjacent parent material should bear semblance to the unwelded parent material- having a bright silver or a light straw color. Blue-gray or gray discoloration- or the presence of loose scale is however NOT acceptable.
In regards to preventative measures like others have mentioned, a lower pressure in the vacuum chamber will help- at least 8 x 10-5 mbar. Another preventive practice would be waiting at least ten minutes after welding is complete before venting the chamber. Depending on the nature or configuration of these welds, using EB welders with CNC controlled multi-axis fixtures (versus manual controls) could possibly reduce the number of times this part would have to be taken in and out of the chamber. Multiple welds could be performed in one pumpdown.

Here is a link to one of our case studies of an aerospace application using the same material:
http://www.ptreb.com/Industries/Aerospace/Turbine_Blade_Repair_Welding/